1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus wherein a photosensitive drum is exposed to light so as to obtain an electrostatic latent image thereon.
2. Description of Related Art
As conventional image forming apparatuses, for example, an image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 10-309826 (Reference 1) and an image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2002-331702 (Reference 2) are well known. In the image forming apparatus disclosed by Reference 1, in order to form an electrostatic latent image, light emitted from an LED array is imaged on the circumferential surface of a photosensitive drum by use of a lens array. This lens array is composed of a plurality of rod lenses that are arranged in two lines extending in a main-scanning direction.
In the image forming apparatus disclosed by Reference 1, however, it is very difficult to speed up the formation of an electrostatic latent image. More specifically, since the rod lenses are arranged in two lines extending in the main-scanning direction, most part of the light emitted from the LED array does not enter into the rod lenses and leaks out from the effective area of the rod lenses, with respect to a sub-scanning direction. Accordingly, the quantity of light used to form an electrostatic latent image is small, and speedy formation of an electrostatic latent image is difficult.
In the image forming apparatus disclosed by Reference 2, a lens array is composed of rod lenses that are arranged in three lines extending in a main-scanning direction. Further, an LED array is displaced from the center of the lens array in a sub-scanning direction so that the beam profile during scanning on the photosensitive drum will not vary.
In the image forming apparatus disclosed by Reference 2, however, there is a problem that an electrostatic latent image formed therein has poor contrast. More specifically, since the LED array is displaced from the center of the lens array in the sub-scanning direction, the angle of field of light to the rod lenses in the line farthest from the LED array is larger than the angle of field of light to the rod lenses in the other two lines. Then, also suffering from the effect of field curvature of the rod lenses, the image points of the rod lenses in the line farthest from the LED array are displaced from the image points of the rod lenses in the other two lines, and consequently, the electrostatic latent image formed thereby has poor contrast.